Gas sensor element, gas sensor, and production method thereof

ABSTRACT

A gas sensor element includes an insulating ceramic base, a solid electrolyte body, and a heating element. The solid electrolyte body is disposed in an opening of the insulating ceramic base and has a measuring electrode affixed to one of major surfaces thereof and a reference electrode affixed to the other major surface. The measuring electrode is exposed to gas to be measured. The reference electrode is exposed to a reference gas. The heating element works to activate the solid electrolyte body and is mounted on one of opposed surfaces of the insulating ceramic base on the same side as the major surface of the solid electrolyte body on which the reference electrode is disposed. Specifically, the insulating ceramic base is located between the solid electrolyte body and the heating element, thereby ensuring a desired degree of electric insulation between the heating element and the reference electrode.

CROSS REFERENCE TO RELATED DOCUMENT

The present application is a divisional of U.S. patent application Ser.No. 13/628,242, filed Sep. 27, 2012, which claims the benefit ofJapanese Patent Application No. 2011-210167 filed on Sep. 27, 2011, theentire content of each of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates generally to a gas sensor element whichmay be installed in an exhaust pipe of an internal combustion engine tomeasure the concentration of a specified component of exhaust emissionsof the engine, and more particularly to a gas sensor equipped with asolid electrolyte body which at least exhibits oxygen ion conductivityand has a pair of electrode layers formed on opposed surfaces thereofand an electrically activated heater. The disclosure also relates to agas sensor equipped with the above type of gas sensor element andproduction method thereof.

2. Background Art

A gas sensor element is known which is disposed in an exhaust pathextending from an internal combustion engine, such as automotive engine,to measure a specified gas component of exhaust emissions, such asoxygen, nitrogen oxide (NOx), ammonia, or hydrogen, for controlling theburning of fuel in the engine or an operation of an exhaust emissioncontrol system.

Japanese Patent First Publication No. H01-253649 discloses the abovetype of gas sensor element equipped with a solid electrolyte body and aheating element stacked on the solid electrolyte body. The solidelectrolyte body has a measurement gas-exposed electrode and a referencegas-exposed electrode. The measurement gas-exposed electrode is formedon a surface of the solid electrolyte body to be exposed to gas to bemeasured (which will also be referred to as a measurement gas below).The reference gas-exposed electrode is also formed on another surface ofthe solid electrolyte body to be exposed to a reference gas chamberfilled with a reference gas. The gas sensor element is produced byfiring a sensor layer and a heater layer to make a laminate of the solidelectrolyte body and the heating element. The heating element works toheat the whole of the solid electrolyte body quickly to activate it.This type of gas sensor element is usually called a planar gas sensorelement.

Japanese Patent First Publication No. 2002-228626 discloses a solidelectrolyte oxygen sensor element which is made of a laminate of asensing portion, an insulating layer, and a heating portion stacked onthe sensing portion through the insulating layer. The heating portion isequipped with a heating element working to activate the sensing portionto measure the concentration of oxygen correctly. The insulating layeravoids the leakage of current from the heating element to the sensingportion.

Japanese Patent Second Publication No. 06-048258 discloses an oxygenconcentration sensor equipped with a hollow insulating ceramic cylinder,an oxygen concentration measuring device, a sheet assembly, and aninsulating protective layer. The insulating ceramic cylinder has aclosed end and an open end. The insulating ceramic cylinder also hasdefined therein a reference gas chamber leading to the open end. Theinsulating ceramic cylinder also has an opening formed in a peripheralwall thereof in communication with the reference gas chamber. The oxygenconcentration measuring device is fit in the opening of the insulatingceramic cylinder and equipped with electrodes affixed to opposedsurfaces thereof. One of the electrode faces outwardly of the insulatingceramic cylinder, while the other electrode faces inwardly of theinsulating ceramic cylinder. The sheet assembly is made up of aninsulating sheet and heater leads and electrode leads affixed to theinsulating sheet. The heater leads and the electrode leads are formed bymetallic films. The insulating sheet also has an opening. The sheetassembly is wrapped around the periphery of the insulating ceramiccylinder with the opening facing the oxygen concentration measuringdevice. The protective layer is porous and disposed over the closed endand the opening of the insulating ceramic cylinder. The insulatingceramic cylinder and the sheet assembly are fired together. One of theopposed surfaces of the oxygen concentration measuring device is exposedto the reference gas chamber, while the other surface is exposed to thegas to be measured through the protective layer.

FIGS. 8( a), 8(b), and 8(c) illustrates a gas sensor element 10 g, likein Japanese Patent First Publication No. H01-253649. The gas sensorelement 10 g will also be described later as a comparative example No.1.

The gas sensor element 10 z is of a planar type and includes a heatingelement 140 z, and a solid electrolyte layer 100, and a reference gaschamber 130 z formed between the heating element 140 z and the solidelectrolyte layer 100. Air, which is highly electrically insulating, isadmitted into the reference gas chamber 130 z. The air in the referencegas chamber 130 z obstructs transmission of heat, as produced by theheating element 140 z, to the solid electrolyte layer 100 z, thusresulting in a lag in activating the solid electrolyte layer 100 z tomeasure the gas correctly.

The gas sensor element 10 z is in the form of a planar plate which istypically susceptible to breakage due to thermal stress. It is, thus,necessary to increase the thickness of insulating layers 150 z and 160 zin order to improve the durability of the gas sensor element 10 z. This,however, results in an increase in overall size of the gas sensorelement 10 z, which leads to a drop in thermal efficiency and anincreased lag in activating the gas sensor element 10 z.

The oxygen gas sensor, as taught in the above described Japanese PatentFirst Publication No. 2002-228626, has the insulating layer between thesensing portion and the heating portion. The insulating layer is formedby firing a green sheet or using screen printing techniques.

Thinning the insulating layer in a production process thereof may causedefects such as pinholes to be developed. The measurement gas, thus,passes through the pinholes and reaches the heating element. The heatingelement may react with contaminants in the measurement gas and thensublimate, thus resulting in a deterioration thereof. Increasing thethickness of the insulating layer in order to increase the resistance ofthe heating element to the oxidation for ensuring a required lifespanthereof will result in an increase in overall size of the oxygen gassensor. Lots of thermal energy is, thus, needed to heat the insulatinglayer. In other words, lots of time is consumed to heat and activate thesolid electrolyte layer.

The oxygen concentration sensor, as taught in the above describedJapanese Patent Second Publication No. 06-048258, is equipped with theinsulating ceramic cylinder with the opening formed in the peripheralwall thereof. The opening has an inner shoulder serving as a seat onwhich the oxygen concentration measuring device made of a solidelectrolyte body is fit. The oxygen concentration measuring device hasthe electrodes affixed to the opposed surfaces thereof. Such anarrangement of the oxygen concentration measuring device results incomplexity of layout of the electrode leads, which may lead to cracks inthe electrode leads, the insulating ceramic cylinder, and the solidelectrolyte body when being fired.

Japanese Patent Second Publication No. 06-048258 also teaches theinsulating sheet made up of two discrete sheets: one being a heatercarrier sheet on which the heating element is formed, and the otherbeing an electrode carrier sheet on which the heater leads and theelectrode leads are formed. The heater carrier sheet and the electrodecarrier sheet are affixed separately to the insulating ceramic cylinder,thus resulting in a lack in transmitting the thermal energy produced bythe heating element to the electrode carrier sheet. This leads to a lagin activating the oxygen concentration sensor. Additionally, it is alsonecessary to affix the heater carrier sheet and the electrode carriersheet to the insulating ceramic cylinder so as not to overlap eachother. This contributes to inconvenience in production of the oxygenconcentration sensor.

SUMMARY

It is therefore an object of the disclosure to provide a gas sensorelement which works to measure a specified component of gas and designedto have a quickly activatable/easy-to-manufacture structure and/orexhibit an increased degree of durability.

It is another object of the disclosure to provide a gas sensor equippedwith the above type of gas sensor element and a production method of thegas sensor element.

According to one aspect of the invention, there is provided a gas sensorelement which may be employed in automotive vehicles to measure theproportion of oxygen (O₂) in exhaust gas emitted from an internalcombustion engine for control of an air-fuel ratio in the engine.

The gas sensor element comprises: (a) an insulating ceramic member whichhas surfaces opposed to each other and a through hole formed therein;(b) a solid electrolyte body which is disposed in the hole of theinsulating ceramic member and works to conduct to at least a given ion,the solid electrolyte body having a first major surface and a secondmajor surface; (c) a measuring electrode disposed on the first majorsurface of the solid electrolyte body to be exposed to the gas; (d) areference electrode disposed on the second major surface of the solidelectrolyte body to be exposed to a reference gas; and (e) a heatingelement disposed on one of the opposed surfaces of the insulatingceramic member on the same side as the second major surface of the solidelectrolyte body. The heating element works to activate the solidelectrolyte body.

Specifically, the insulating ceramic member, which is highlyelectrically insulating is disposed between the solid electrolyte bodyand the heating element, thereby ensuring a desired degree of electricinsulation between the heating element and the reference electrode. Thisminimizes the leakage of current from the heating element to thereference electrode to ensure the stability in operation of the gassensor element.

When the heating element is actuated, the thermal energy produced by theheating element is transmitted to the solid electrolyte body through theinsulating ceramic member, which has a high thermal conductivity, thusaccelerating the activation of the solid electrolyte body andestablishing the stability in operation of the gas sensor elementquickly.

The heating element is disposed on the surface of the insulating ceramicmember on the same side as the second major surface of the solidelectrolyte body. The insulating ceramic member, thus, serves as amechanical support for the heating element and a protective or shieldmember for isolating the heating element from the gas. This structureimproves the service life of the heating element and permits the overallsize of the gas sensor element to be reduced.

In the preferred mode of the embodiment, the gas sensor element mayfurther include a hollow cylindrical ceramic member which has a closedend and defines therein a reference gas chamber into which the referencegas is admitted. The hollow cylindrical ceramic member also has formedin a peripheral surface thereof a window which communicates with thereference gas chamber. The insulating ceramic member is stacked on thehollow cylindrical member with the solid electrolyte body exposed to thereference gas chamber through the window. The heating member isinterposed between the hollow cylindrical ceramic member and theinsulating ceramic member.

In other words the heating element is covered with the insulatingceramic member and the cylindrical ceramic member and thus protectedfrom the gas and the reference gas, thereby enhancing the stability inoperation of the gas sensor element. Additionally, use of thecylindrical ceramic member also improves the mechanical strength of thegas sensor element and resistance to thermal stress breakage arisingfrom being splashed with, for example, water.

The heating element may be located at a given insulating interval awayfrom one of the solid electrolyte body and the reference electrode. Inother words, the heating element is away from the reference electrode aswell as the solid electrolyte body, thus enables the solid electrolytebody to be activated quickly without sacrificing the electric insulationfrom the heating element. The insulating interval may be defined as adistance which is the shorter of a minimum distance between a peripheraledge of the heating element and a peripheral edge of the solidelectrolyte body and a minimum distance between the peripheral edge ofthe heating element and a peripheral edge of the reference electrode andgreater than or equal to 0.1 mm and smaller than or equal to 3 mm.

In the case where the insulating interval is less than 0.1 mm, thecurrent may leak from the heating element to the solid electrolyte bodyand/or the reference electrode, which results in instability inoperation of the gas sensor element. Alternatively, in the case wherethe insulating interval is greater than 3.0 mm, the overall size of thegas sensor element is increased. It, therefore, takes much time toactivate the solid electrolyte body through the heating element.

The solid electrolyte body may be made of a partially-stabilizedzirconia, thereby enhancing quick activation of the solid electrolytebody and improving the durability of the gas sensor element.

The insulating ceramic member may be made of alumina. The use of aluminaenhances the electric insulation and thermal conductivity, therebyenhancing the quick activation of the solid electrolyte body further.

According to the second aspect of the embodiment, there is provided agas sensor which works to measure a given component of gas whichcomprises: (1) a gas sensor element including (a) an insulating ceramicmember which has surfaces opposed to each other and a through holeformed therein, (b) a solid electrolyte body which is disposed in thehole of the insulating ceramic member and works to conduct to at least agiven ion, the solid electrolyte body having a first major surface and asecond major surface, (c) a measuring electrode disposed on the firstmajor surface of the solid electrolyte body to be exposed to the gas,(d) a reference electrode disposed on the second major surface of thesolid electrolyte body to be exposed to a reference gas, and (e) aheating element disposed on one of the opposed surfaces of theinsulating ceramic member on the same side as the second major surfaceof the solid electrolyte body, the heating element working to activatethe solid electrolyte body; (2) a first and a second signal line leadingto the reference electrode and the measuring electrode for transmittinga sensor output to an external detection circuit; (3) a first and asecond conductor leading to the heating element for establishingelectric connections with an external power supply control circuit tocontrol supply of electric power to the heating element; and (4) ahousing in which the gas sensor element, the first and second signalline, and the power supply conductors are retained. The housing isdesigned to hold the gas sensor element to be exposed to the gas.

The above structure of the gas sensor is capable of activating the gassensor element quickly and ensures the stability in operation andmechanical durability of the gas sensor element. The structure is alsoeasy to assemble and permits the size thereof to be reduced.

According to third aspect of the embodiment, there is provided a gassensor element production method which comprises: (a) forming aninsulating ceramic member which is planar and made of an electricallyinsulating ceramic material, the insulating ceramic member havingopposed surfaces and a through hole; (b) forming a solid electrolytebody in the through hole of the insulating ceramic member, the solidelectrolyte body being made of a ceramic material which conducts atleast a given ion and having a first major surface and a second majorsurface; (c) forming a measuring electrode disposed on the first majorsurface of the solid electrolyte body to be exposed to the gas; (d)forming a reference electrode disposed on the second major surface ofthe solid electrolyte body to be exposed to a reference gas; (e)arranging a heating element disposed on one of the opposed surfaces ofthe insulating ceramic member on the same side as the second majorsurface of the solid electrolyte body, the heating element working toactivate the solid electrolyte body; (f) preparing a hollow cylindricalceramic member which is made of an electrically insulating ceramicmaterial and has a closed end and an open end, the hollow cylindricalceramic member also having a window formed in a peripheral surfacethereof; (g) wrapping the insulating ceramic member around the hollowcylindrical ceramic member with the reference electrode facing thewindow of the hollow cylindrical ceramic member; and (h) firing thehollow cylindrical ceramic member around which the insulating ceramicmember is wrapped.

The above production method achieves quick activation of the gas sensorelement and improves the durability thereof.

The forming steps of the insulating ceramic member and the solidelectrolyte body may prepare a stack of a planar solid electrolytematerial and a planer insulating ceramic material and punch the stack tomake the through hole in the insulating ceramic member and the solidelectrolyte body cut from the planar solid electrolyte material to havea size substantially identical with that of the through hole andsimultaneously to place the solid electrolyte body in the through hole,thereby simplifying the production process of the gas sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinbelow and from the accompanying drawings of thepreferred embodiments of the invention, which, however, should not betaken to limit the invention to the specific embodiments but are for thepurpose of explanation and understanding only.

In the drawings:

FIG. 1( a) is a partially longitudinal sectional view which shows a gassensor element according to an embodiment;

FIG. 1( b) is a perspective view which shows a heating element installedin the gas sensor element of FIG. 1( a);

FIG. 1( c) is a partially transverse sectional view, taken along theline A-A in FIG. 1( a);

FIG. 1( d) is a view which represent a temperature distribution in thegas sensor element of FIG. 1( c);

FIG. 2 is a longitudinal sectional view which illustrates a gas sensorequipped with the gas sensor element of FIGS. 1( a) to 1(d);

FIGS. 3( a-1), 3(b-1), 3(c-1), and 3(d-1) are partially longitudinalsectional views which illustrate a sequence of punching step ofproducing an assembly of a solid electrolyte body and an insulatingceramic base which is installed in the gas sensor element of FIGS. 1( a)to 1(d);

FIGS. 3( a-2), 3(b-2), 3(c-2), and 3(d-2) are partially perspectiveviews which illustrate products in the punching steps, as illustrated inFIGS. 3( a-1), 3(b-1), 3(c-1), and 3(d-1), respectively;

FIG. 4 is an exploded perspective view which illustrates the gas sensorelement of FIGS. 1( a) to 1(d);

FIG. 5( a) is a plane view which illustrates a surface of an insulatingceramic base of the gas sensor element of FIGS. 1( a) to 1(d) on which ameasuring electrode is disposed;

FIG. 5( b) is a longitudinal sectional view which of FIG. 5( a);

FIG. 5( c) is a plane view which illustrates a surface of an insulatingceramic base of the gas sensor element of FIGS. 1( a) to 1(d) on which areference electrode and a heating element are disposed;

FIG. 5( d) is a sectional view which shows how to wrap the insulatingceramic base of FIGS. 5( a) to 5(c) around a cylindrical ceramic base;

FIG. 6( a) is a partially plane view which illustrates a surface of aninsulating ceramic base of a modified form of a gas sensor element onwhich a heating element and a reference electrode are disposed;

FIG. 6( b) is a partially longitudinal sectional view which of FIG. 6(a);

FIG. 6( c) is a partially plane view which illustrates a surface of aninsulating ceramic base of the gas sensor element of FIGS. 6( a) and6(b) on which a measuring electrode is disposed;

FIG. 6( d) is a partially transverse sectional view of the gas sensorelement of FIGS. 6( a) to 6(c);

FIG. 6( e) is a partially transverse sectional view which shows theinsulating ceramic base of FIGS. 6( a) to 6(e) wrapped around acylindrical ceramic base;

FIGS. 7( a) and 7(b) are partially side views which illustrate leadsaffixed to an insulating ceramic base of a modified form of a gas sensorelement;

FIG. 7( c) is a transverse sectional view which illustrates theinsulating ceramic base wrapped around a cylindrical ceramic base andelectric connections with an external power supply control circuit andan external detector circuit;

FIG. 7( d) is a longitudinal sectional view which shows a springconnector for use in establishing electric connections of the gas sensorelement with the external power supply control circuit and the externaldetector circuit of FIG. 7( c);

FIG. 8( a) is an exploded perspective view which shows a comparativeexample No. 1 of a gas sensor element;

FIG. 8( b) is a partially transverse sectional view, as taken along theline A-A in FIG. 8( a);

FIG. 8( c) is a partially transverse sectional view which represent atemperature distribution in the gas sensor element of FIGS. 8( a) and8(b);

FIG. 9( a) is an exploded perspective view which shows a comparativeexample No. 2 of a gas sensor element;

FIG. 9( b) is a partially transverse sectional view of the gas sensorelement of FIG. 9( a);

FIG. 9( c) is a partially transverse sectional view which represent atemperature distribution in the gas sensor element of FIGS. 9( a) and9(b);

FIG. 10( a) is an exploded perspective view which shows a comparativeexample No. 3 of a gas sensor element;

FIG. 10( b) is a partially longitudinal sectional view of the gas sensorelement of FIG. 10( a);

FIG. 10( c) is a transverse sectional view of the gas sensor element ofFIG. 10( a);

FIG. 11( a) is an exploded perspective view which shows a comparativeexample No. 4 of a gas sensor element;

FIG. 11( b) is a partially transverse sectional view of the gas sensorelement of FIG. 11( a);

FIG. 12( a) is an exploded perspective view which shows a comparativeexample No. 5 of a gas sensor element;

FIG. 12( b) is a partially transverse sectional view of the gas sensorelement of FIG. 12( a);

FIG. 13( a) is an exploded perspective view which shows a comparativeexample No. 6 of a gas sensor element;

FIG. 13( b) is a partially transverse sectional view of the gas sensorelement of FIG. 13( a);

FIG. 14( a) is an exploded perspective view which shows a comparativeexample No. 7 of a gas sensor element; and

FIG. 14( b) is a partially transverse sectional view of the gas sensorelement of FIG. 14( a).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numbers refer tolike parts in several views, particularly to FIGS. 1( a) to 1(d), thereis shown a gas sensor element 10 according to the first embodiment.

The gas sensor element 10 may be mounted in a so-called A sensor whichmeasures the proportion of oxygen (O₂) in exhaust gas (which will alsobe referred to as measurement gas below) emitted from an internalcombustion engine or a gas sensor for use in measuring the concentrationof nitrogen oxide (NOx), sulfur oxide (SOx), carbon hydride (HC), orcarbon monoxide (CO) contained in the exhaust gas for control of anair-fuel ratio in the engine.

The following discussion will refer to the gas sensor element 10 asbeing made of a solid electrolyte material having an oxygen ionconductivity installed in, for example, an oxygen sensor.

The gas sensor element 10, as illustrated in FIGS. 1( a) and 1(b), ismade up of a sensor/heater laminate sheet 20 and a bottomed hollowcylindrical ceramic base 13. The sensor/heater laminate sheet 20 iswrapped about the cylindrical ceramic base 13 and made up of a solidelectrolyte body 100, a measuring electrode 110, a measuring electrodelead 111, a measuring electrode terminal 112, a reference electrode 120,a reference electrode lead 121, a reference electrode terminal 122, aheating element 140, heater leads 141 and 142, power supply terminals145 and 146, a measurement gas chamber 170, a diffusion resistance layer180, a slant measurement gas inlet surface 181, a shield layer 190, abuffer layer 191, and an insulating ceramic base 200. The cylindricalceramic base 13 is made up of a reference gas chamber 130, a peripheralside wall 131, a through hole 132, and a closed end 133.

The solid electrolyte body 100 has opposed major surfaces. The measuringelectrode 110 which is to be exposed to the measurement gas is disposedon one of the major surfaces of the solid electrolyte body 100. Thereference electrode 120 which is to be exposed to air admitted into thegas sensor element 10 as a reference gas is mounted on the other majorsurface of the solid electrolyte body 100. The solid electrolyte body100, the measuring electrode 110, and the reference electrode 120constitute a sensing mechanism which will also be referred to as asensing portion below.

The insulating ceramic base 200 is made from a ceramic material such asalumina having electric insulation properties. The insulating ceramicbase 200 is identical in thickness with the solid electrolyte body 100.The solid electrolyte body 100 is in the form of a layer and fit in athrough hole (i.e., a window) 201 formed in the insulating ceramic base200. The major surface of solid electrolyte body 100 may be laid flushwith, slightly protrude, or slightly sinks from the surface of theinsulating ceramic base 200.

The cylindrical ceramic base 13 is made of a ceramic material such asalumina having electric insulation properties and has the closed end 133and an open end 134 opposed to the closed end 133. The cylindricalceramic base 13 has defined therein the reference gas chamber 130 intowhich the air is admitted as the reference gas. The cylindrical ceramicbase 13 also has the through hole 312 formed in a portion of the sidewall 131 which is closer to the closed end 133 than to the open end 134.

The insulating ceramic base 200 is wrapped about the outer periphery ofthe cylindrical ceramic base 13. The holes 201 and 132 are incoincidence with each other. The reference electrode 120 disposed on thesurface of the solid electrolyte body 100 faces the hole 132 and isexposed directly to the air in the reference gas chamber 130.

The measuring electrode 110 disposed on the solid electrolyte body 100faces the measurement gas chamber 170. Specifically, the measurement gaschamber 170 surrounds the whole of a major surface of the measuringelectrode 110. The measuring electrode 110 is exposed directly to thereference gas in the reference gas chamber 170.

The measuring electrode 110 and the reference electrode 120 are coupledto the measuring electrode lead 111 and the reference electrode lead121, respectively, which connect with an external power supply (notshown) and a detector circuit or a controller (not shown).

The heating element 140 is, as clearly illustrated in FIG. 4, of asubstantially C-shape and affixed to the major surface of the insulatingceramic base 200 which lies flush with the reference electrode 120 onthe solid electrolyte body 100. The heating element 140 is, as can beseen in FIG. 1( c), located at an electrically insulating interval daway from the reference electrode 120 and embraces, as illustrated inFIG. 4, the periphery of the measuring electrode 110. The solidelectrolyte body 100 may have a width greater than that of the referenceelectrode 120 in a circumferential direction of the gas sensor element10. In this case, the insulating interval d is a distance between theheating element 140 and the solid electrolyte body 100. In other words,the insulating interval d is the shorter of a minimum distance betweenthe inner edge of the heating element 140 and the outer edge of thereference electrode 120 and a minimum distance between the inner edge ofthe heating element 140 and the outer edge of the solid electrolyte body100.

The heating element 140 is interposed between bonded surfaces of thecylindrical ceramic base 13 and the insulating ceramic base 200.

The heating element 140 is connected at ends thereof to the heater leads141 and 142 which are joined to the external power supply and anenergization controller (which is not shown and will also be referred toas a power supply control circuit below) through heater terminals 143and 144. When energized, the heating element 140 produces thermal energyto elevate the temperature of the sensing mechanism (i.e., the solidelectrolyte body 100, the measuring electrode 110, and the referenceelectrode 120) up to a value at which the sensing mechanism is placed ina desirable activated state.

The diffusion resistance layer 180 is made of a porous material having agiven diffusion resistance and disposed over the whole of themeasurement gas chamber 170. The shield layer 190 is affixed to coverthe entire outer surface of the diffusion resistance layer 180 exceptthe slant measurement gas inlet surface 181.

The slant measurement gas inlet surface 181 is formed by a tapered endof the diffusion resistance layer 180 which is exposed directly outsidethe gas sensor element 10 and through which the measurement gas isadmitted into the measurement gas chamber 170.

The shield layer 190 and the buffer layer 191 serve to shield themeasuring electrode lead 111 from the measurement gas and also to avoidleakage of the measurement gas flowing through the diffusion resistancelayer 180 to the outside of the gas sensor element 10.

Tests were performed as will be described later in detail, and thefollowing facts were found.

The solid electrolyte body 100 is, as described above, embedded in theinsulating ceramic base 200. The heating element 140 is disposed on themajor surface of the insulating ceramic base 200 which lies flush withthe reference electrode 120 on the solid electrolyte body 100, has theperiphery located at the electrically insulating interval d (preferablygreater than or equal to 0.1 mm and less than or equal to 3 mm) awayfrom the periphery of the solid electrolyte body 100, and embraces theperiphery of the solid electrolyte body 100. Note that when thereference electrode 120 is greater in size than the solid electrolytebody 100, the insulating interval d is, as described above, a distancebetween the heating element 140 and the reference electrode 120. Whenthe heating element 140 is energized, the solid electrolyte body 100 is,as illustrated in FIG. 1( c), heated directly by the thermal energy, astransmitted through the insulating ceramic base 200, and activatedquickly.

The partially-stabilized zirconia that is material of the solidelectrolyte body 100 is low in thermal conductivity (i.e., 2 to 3W/m·K). A range in which the thermal conductivity is low is minimizedaround the measuring electrode 110 and the reference electrode 120 byembedding the solid electrolyte body 100 in the insulating ceramic base200 which is high in thermal conductivity (i.e., 20 to 30 W/m·K),thereby resulting in an increased degree of thermal conductivity of thewhole of the gas sensor element 10. This accelerates the rise intemperature of the gas sensor element 10. The alumina that is materialof the insulating ceramic base 200 is high in electric insulation servesto keep the leakage of electric current to the sensing mechanism lowwhen the heating 140 is energized even if the insulating interval d isshortened. This enables the heating element 140 to be located close tothe solid electrolyte body 100, thus resulting in a decreased timerequired to activate the solid electrolyte body 100.

The heating element 140 is, as described above, interposed between theinsulating ceramic base 200 and the cylindrical ceramic base 13, inother words, shielded from the measurement gas, thus avoiding thedeterioration thereof arising from poisons contained in the measurementgas and ensuring the stability in operation of the gas sensor element10.

If the heating element 140 is affixed to the surface of the insulatingceramic base 200 with which the measuring electrode 110 is flush, itresults in need for a protective layer to isolate the heating element140 from the measurement gas, which leads to an increase in overall sizeof the gas sensor element 10. The above structure of the gas sensorelement 10 eliminate the need for such a protective layer, thuspermitting the size of the gas sensor element 10 to be reduced.

The ceramic base 13 is of a cylindrical shape and thus greater inmechanical strength than the conventional structure in which the gassensor element 10 is of a planar shape, thus exhibiting the durabilitygreat enough to withstand thermal impact arising from being splashedwith water.

The gas sensor element 10 has been described as being installed in thegas sensor designed to measure the concentration of oxygen (O₂), buthowever, may be engineered to measure another kind of gas. The gassensor element 10 may alternatively be made by using ABO₃-typetransition metal oxide such as SrZrO₃ or SrC3O₃ having protonconductivity as material of the solid electrolyte body 100, tungstencarbide, silicon nitride, or ruthenium oxide as material of the heatingelement 140, or titania or spinel as material of the insulating ceramicbase 200.

The surface of the gas sensor element 10 may be covered with a porousprotective layer which is formed by heat-resisting ceramic particlesusing dipping or plasma spraying techniques for minimizing the risk ofbreakage due to being splashing with water or deterioration due to beingsubjected to poisons.

FIG. 2 illustrates a gas sensor 1 in which the gas sensor element 10 ismounted.

The gas sensor element 10, as described above, has the sensor/heaterlaminate sheet 20 wrapped about the cylindrical ceramic base 13. Thecylindrical ceramic base 13 is oriented in the gas sensor 1 with theclosed end 133 facing the top end (i.e., the head) of the gas sensor 1and the open end 134 facing the base end (i.e., an upper end, as viewedin FIG. 2) of the gas sensor 1. The cylindrical ceramic base 13 isdisposed inside a hollow cylindrical insulator 41 made from anelectrically insulating ceramic material such as alumina and retainedfirmly therein by a heat-resisting bond 40 such as ceramic cement orheat-resisting glass. The assembly of the ceramic base 13 and theinsulator 41 (i.e., the gas sensor element 10) is disposed in a hollowcylindrical housing 30. The gas sensor element 10 has a top end portionwhich protrudes outside the housing 30 and is exposed to the measurementgas. The top end portion works as a sensing portion which is sensitiveto the measurement gas.

The insulator 41 is retained firmly inside the housing 30 through asealant 42 such as talc. The housing 30 is made of a hollow cylindricalmetallic member such as stainless steel.

The housing 30 has a top end and a base end. A cup-shaped double-walledprotective cover assembly is secured to the top end of the housing 30.The cover assembly is made up of a bottomed inner cover 50 and abottomed outer cover 60 enclosing the inner cover 50 coaxially. Theinner cover 50 and the outer cover 60 have base ends shaped into flanges51 and 61. The flanges 51 and 61 are grasped firmly by elasticallybending or crimping a cylindrical extension 35 (which will also bereferred to as a crimped portion below) formed on the top end of thehousing 30 to make a firm joint of the cover assembly to the housing 30.

The gas sensor 1 also include a hollow cylindrical casing 47 made of ametallic material such as stainless steel. The casing 47 is fit on aboss 31 formed on the base end of the housing 30 and holds thereinsignal lines 81 and 82 and power supply lines (which will also bereferred to as power supply conductors below) 83 and 84 to be insulatedfrom each other through an insulator 43. The signal lines 81 and 82 andthe power supply lines 83 and 84 are also retained air-hermetically inthe casing 47 through a sealing rubber 46, a water-repellent filter 45,and a support 44. The water-repellent filter 45 is fit on the support44. The signal lines 81 and 82 and the power supply lines 83 and 84 areelectrically joined through connecting terminals 113, 123, 147, and 148and metallic connectors (e.g., crimping terminals) 114, 124, 149, and150 to the measuring electrode terminal 112, the reference electrodeterminal 122, and the power supply terminals 145 and 146, respectively,which extend from the base end of the gas sensor element 10. The signallines 81 and 82 are used to transmit a sensor output to the detectorcircuit, as described above. The power supply lines 83 and 84 are usedto supply electric power from the external power supply to the heatingelement 140.

The casing 47 has reference gas inlet holes 471 formed in a side wallthereof. The sealing rubber 46 has reference gas inlet holes 461 formedthrough a side wall thereof. The support 44 also has reference gas inletholes 441 formed through a side wall thereof. The reference gas inletholes 471, 461, and 441 communicate with each other to define areference gas inlet path through which the air (i.e., the reference gas)is admitted into the reference gas chamber 130. The water-repellentfilter 45 serves to prevent water or moisture from entering thereference gas chamber 130.

The inner and outer covers 50 and 60 are, as described above, of acup-shape and have bottom surfaces 54 and 64 (i.e., top end surfaces inFIG. 2), respectively. The inner and outer covers 50 and 60 are laidcoaxially with each other to form the double-walled protective coverassembly. The inner cover 50 has gas inlet holes 52 and 55 formed in aside surface 53 and the bottom surface 54. Similarly, the outer cover 60has gas inlet holes 62 and 65 formed in a side surface 63 and the bottomsurface 64. The gas inlet holes 52, 55, 62, and 65 work to control thevelocity of the measurement gas flowing into or outside the coverassembly of the inner and outer covers 50 and 60. The top end portion(i.e., the sensing portion) of the gas sensor element 10 is exposed tothe measurement gas within the cover assembly to produce an output as afunction of the concentration of, for example, O₂ of the measurementgas.

The housing 30 has an external thread 34 formed on a top end portionthereof. The thread 34 is fastened into a wall of a gas flow pipe 70(i.e., an exhaust pipe extending from an internal combustion engine) tohave the sensing portion of the gas sensor element 10 exposed to themeasurement gas 700.

The insulator 41, the housing 30, the casing 47, and the inner and outercovers 50 and 60 are not limited to the above structures, but may bedesigned to have another known structures, respectively.

In an operation of the gas sensor 1, when supplied with the electricpower through the external power supply control circuit, the heatingelement 140 produces heat which is, in turn, transmitted to the solidelectrolyte body 100 through the insulating ceramic base 200, so thatthe solid electrolyte body 100 is activated. Upon activation of thesolid electrolyte body 100, a potential difference will be developedbetween the measuring electrode 110 and the reference electrode 120 as afunction of a difference in concentration of oxygen (O₂) between themeasurement gas which has been introduced into the measurement gaschamber 170 through the diffusion resistance layer 180 and the referencegas (i.e., the air) which has been introduced into the reference gaschamber 130. The potential difference is outputted to the detectorcircuit (not shown) through the signal lines 81 and 82 as representingthe concentration of oxygen in the measurement gas. Alternatively, thedetector circuit may apply the voltage across the measuring electrode110 and the reference electrode 120 and monitor a resulting currentflowing through the solid electrolyte body 100 as a function of theconcentration of oxygen in the measurement gas.

A production method of the gas sensor element 10 will be described belowwith reference to FIGS. 3( a-1) to 5(d).

The insulating ceramic base 200 made of an insulating ceramic sheet isprepared. A window is drilled in the insulating ceramic base 200 to makethe hole 201. The solid electrolyte body 100 is embedded or fit in thehole 201. The measuring electrode 110 and the reference electrode 140are affixed to the opposed major surfaces of the solid electrolyte body100. The sensor/heater laminate sheet 20 equipped with the insulatingceramic base 200 and the heating element 140 affixed to the surface ofthe insulating ceramic base 200 with which the reference electrode 120lies flush is glued to the periphery of the cylindrical ceramic base 13in which the reference gas chamber 130 is formed. The sensor/heaterlaminate sheet 20 is, as apparent from the above discussion, equippedwith a sensor function and a heater function and includes at least thesolid electrolyte body 100, the measuring electrode 110, the referenceelectrode 120, the heating element 140, and the insulating ceramic base200. Specifically, the sensor/heater laminate sheet 20 formed by atleast two sheets (i.e., the insulating ceramic base 200 and the shieldlayer 190) is adhered to the cylindrical ceramic base 13 and then firedto make the gas sensor element 10. The sensor/heater laminate sheet 20may alternatively be formed by a single sheet. This is accomplished bywrapping a sheet of the insulating ceramic base 200 in which the solidelectrolyte body 100 is fit around the cylindrical ceramic base 13 andthen forming on the insulating ceramic base 200 the heating element 140,the measuring electrode 110, the reference electrode 120, the measuringgas chamber 170, etc., using coating, plating, or thermal sprayingtechniques without use of the shield layer 190.

The electrode pattern (i.e., the measuring electrode 110, the referenceelectrode 120, the heating element 140, etc.) is formed on the planarceramic sheet (i.e., the insulating ceramic base 200), thus minimizingthe probability of electric disconnections or bonding defects thereof.This improves the reliability in operation of the gas sensor element 10.

The production method of the gas sensor element 10 will also bedescribed below in more in detail.

The solid electrolyte body 100 is made from a solid electrolyte materialcontaining, for example, a main component of zirconia (i.e., zirconiumdioxide ZrO₂) and an additive of yttria (i.e., Y₂O₃, 4-8 mol %).

The solid electrolyte material may also contain alumina, silica,magnesia, and/or calcia. These auxiliary agents serve to improvesintering performance of zirconia, bring the degree of shrinkage (alsocalled contraction percentage) or coefficient of thermal expansion ofthe solid electrolyte material into agreement with those of the materialof the insulating ceramic base 200, or enhance the strength of adhesionamong the solid electrolyte body 100, the insulating ceramic base 200,the cylindrical ceramic base 13, the measuring electrode 110, and thereference electrode 120.

The insulating ceramic base 200 is made from an insulating ceramicmaterial which preferably contains, for example, a main component of 90wt % or more of alumina (i.e., aluminum oxide Al₂O₃) that is high inthermal conductivity and electric insulation. The alumina may containzirconia, yttria, magnesia, calcia, and/or silica. These auxiliaryagents serve to improve sintering performance of alumina, bring thedegree of shrinkage or coefficient of thermal expansion of the aluminainto agreement with those of material of the solid electrolyte body 100,or enhance the strength of adhesion among the insulating ceramic base200, the cylindrical ceramic base 13, the solid electrolyte body 100,the measuring electrode 110, the measuring electrode lead 111, themeasuring electrode terminal 112, the reference electrode 120, thereference electrode lead 121, and the reference electrode terminal 122.

First, an unfired ceramic sheet SH₂₀₀ (i.e., an unfired alumina sheet)which will be the insulating ceramic base 200 is produced. The unfiredceramic sheet SH₂₀₀ will also be referred to as a planer insulatingceramic material or an alumina sheet below.

The alumina sheet SH₂₀₀ is formed by blending or combining aluminapowder with sintering additive such as magnesia powder, binder such asbutyral resin, and plasticizer such as BBP (butyl benzyl phthalate) tomake alumina slurry, shaping the alumina slurry using a doctor bladeinto a sheet, and then volatilizing organic solvent therefrom.

The hole 201 in which the solid electrolyte body 100 is to be fit isformed in the alumina sheet SH₂₀₀. Through holes 202, 203, and 204 arealso drilled in the alumina sheet SH₂₀₀ to make the via-conductors 144,122, and 143. The via-conductor 122 is the reference electrode terminal,as described above, and will also be referred to as a referenceelectrode via-conductor below.

An unfired solid electrolyte sheet SH₁₀₀ (i.e., an unfired zirconiasheet) which will be the solid electrolyte body 100 is produced. Theunfired solid electrolyte sheet SH₁₀₀ will also be referred to as aplanar solid electrolyte material or a zirconia sheet below.

The zirconia sheet SH₁₀₀ is formed by blending or combining zirconiapowder with yttria powder, binder such as butyral resin, and plasticizersuch as BBP (butyl benzyl phthalate) and mixing organic solvent with itto made zirconia slurry, shaping the zirconia slurry using a doctorblade into a sheet, and then volatilizing organic solvent therefrom.

The alumina sheet SH₂₀₀ and the zirconia sheet SH₁₀₀ are identical inthickness with each other (e.g., a thickness of 200 μm after fired).Grain size distributions and blend ratios of the compositions of thealumina sheet SH₂₀₀ and the zirconia sheet SH₁₀₀ are regulated to matchshrinkage ratios thereof with each other when they are fired.

The zirconia sheet SH₁₀₀ is punched into a size and shape (e.g., arectangular shape) identical with those of the through hole 201 formedin the alumina sheet SH₂₀₀. The punched out portion of the zirconiasheet SH₁₀₀ is embedded in the hole 201.

How to embed the punched out portion of the zirconia sheet SH₁₀₀ in thehole 201 of the alumina sheet SH₂₀₀ will be described below in detailwith reference to FIGS. 3( a-1) to 3(d-2).

The zirconia sheet SH₁₀₀ and the alumina sheet SH₂₀₀ are, as illustratedin FIGS. 3( a-1) and 3(a-2), overlaid on each other and then placed in apunch press equipped with an upper die D_(UP), an upper punch P_(UP), alower die D_(LO), a lower punch P_(LO), and a base BS.

The upper die D_(UP) is, as illustrated in FIGS. 3( b-1) and 3(b-2),moved downward to punch the hole 201 in the alumina sheet SH₂₀₀ and, atthe same time, punch out a portion of the zirconia sheet SH₁₀₀ into theshape contoured to conform with the hole 201. The punched portion of thezirconia sheet SH₂₀₀ will be the solid electrolyte body 100 after beingfired. When the upper die D_(UP) is further moved downward, the punchedout portion of the zirconia sheet SH₁₀₀ is pressed by the lower punchP_(LO) into the hole 201.

The piece WST₂₀₀ of material cut from the alumina sheet SH₂₀₀ to makethe hole 201 is lifted up against the upper punch P_(UP) pushed downwardby an upper spring SP_(UP) and ejected into the upper die D_(UP).

A remaining frame-like portion of the zirconia sheet SH₁₀₀ pushes thelower die D_(LO) downward against a lower spring SP_(LO).

Afterward, the upper die D_(UP) is, as illustrated in FIGS. 3( c-1) and3(c-2), moved upward, the remaining frame-like portion of the zirconiasheet SH₁₀₀ from which the portion of the zirconia sheet SH₁₀₀ which hasbeen embedded in the alumina sheet SH₂₀₀ and will be the solidelectrolyte body 100 is cut is lifted up by the lower die D_(LO). Thepiece WST₂₀₀ of material which is cut from the alumina sheet SH₂₀₀ andforced into the upper die D_(UP) is ejected by the downward moving upperpunch P_(UP) out of the upper die D_(UP).

In the manner, as described above, a solid electrolyte body/aluminasheet 20′ that is an assembly of the alumina sheet SH₂₀₀ and the punchedout portion of the zirconia sheet SH₁₀₀ which, after being fired,becomes the sensor/heater laminate sheet 20 is, as illustrated in FIGS.3( d-1) and 3(d-2), produced.

The punch press that is, as can be seen from FIGS. 3( a-1) to 3(d-1),simple in structure is used to press the alumina sheet SH₂₀₀ and thezirconia sheet SH₁₀₀ overlaid on each other to make the hole 201 in thealumina sheet SH₂₀₀ and embed the punched out portion of the zirconiasheet SH₁₀₀ in the hole 201 simultaneously to make the solid electrolytebody/alumina sheet 20′. However, a zirconia sheet which, after beingfired, becomes the solid electrolyte body 100 may be molded in the hole201 of the alumina sheet SH₂₀₀ by putting a zirconia slurry in the hole201 and volatilizing organic solvent therefrom to make the solidelectrolyte body/alumina sheet 20′.

After dried, the zirconia sheet molded in the hole 201, however, usuallyshrinks, so that a central portion thereof is thinned. It is, therefore,essential to make the zirconia sheet whose central thickness is greaterthan the thickness of the rest using the surface tension of the zirconiaslurry so that the thickness of the zirconia sheet after being firedwill be constant.

The solid electrolyte body/alumina sheet 20′ may alternatively be madeby preparing a rectangular zirconia sheet which is similar in shape tothe hole 201, but slightly smaller in size than the hole 201, putting itthe hole 201, and loading a mixture of a zirconia slurry and an aluminaslurry which is diluted by organic solvent into a clearance between thehole 201 and the zirconia sheet put in the hole 201 as an adhesiveagent.

After the solid electrolyte body/alumina sheet 20′ is produced in themanner, as described above, the measuring electrode 110, the measuringelectrode lead 111, the measuring electrode terminal 112, the referenceelectrode 120, the reference electrode lead 121, the reference electrodevia-conductor 122, the reference electrode terminal 123, the heatingelement 140, the heat leads 141 and 142, the heater via-conductors 143and 144, the power supply terminals 145 and 146, the measurement gaschamber 170, the diffusion resistance layer 180, the shield layer 190,and the buffer layer 191 are, as illustrated in FIGS. 4 and 5( d),formed on the solid electrolyte body/alumina sheet 20′ using, forexample, known thick film printing techniques to make the sensor/heaterlaminate sheet 20.

The measuring electrode 110, the measuring electrode lead 111, themeasuring electrode terminal 112, the reference electrode 120, thereference electrode lead 121, the reference electrode via-conductor 122,the reference electrode terminal 123, the heat leads 141 and 142, theheater via-conductors 143 and 144, the power supply terminals 145 and146 may be made from a known conductive material such as gold, platinum,rhodium, palladium, ruthenium, or an alloy thereof. The conductivematerial may contain zirconia that is a main component of the solidelectrolyte body 100 or alumina that is a main component of theinsulating ceramic base 200.

The heating element 140 may be made from a resistance heating materialsuch as platinum, rhodium, tungsten, rhenium, or an alloy thereof. Theresistance heating material may contain alumina that is the maincomponent of the insulating ceramic base 200.

The measuring electrode 110 is, as illustrated in FIGS. 4, 5(a), and5(b), of a substantially rectangular shape and printed over the whole ofone of the major surfaces of the solid electrolyte body 100.

The measuring electrode lead 111 is joined to one of ends of themeasuring electrode 110 which faces the base end of the gas sensorelement 10 and extends in a lengthwise direction of the insulatingceramic base 200 (i.e., the gas sensor element 10). The measuringelectrode lead 111 is printed on one of the major surfaces of theinsulating ceramic base 200 which lies flush with the measuringelectrode 110.

The reference electrode 120 is, as illustrated in FIGS. 4, 5(b), and5(c), of a substantially rectangular shape and printed over the whole ofthe other major surface of the solid electrolyte body 100. In otherwords, the reference electrode 120, the solid electrolyte body 100, andthe measuring electrode 110 are laid to overlap each other in athickness direction thereof.

The reference electrode lead 121 is joined to one of ends of thereference electrode 120 which faces the base end of the gas sensorelement 10 and extends in the lengthwise direction of the insulatingceramic base 200 (i.e., the gas sensor element 10). The measuringelectrode lead 111 is printed on one of the major surfaces of theinsulating ceramic base 200 which lies flush with the referenceelectrode 120.

The via-conductor 122 is formed inside the hole 203 of the insulatingceramic base 200 using known vacuum printing techniques. The hole 203extends through the opposed major surfaces of the insulating ceramicbase 200 on which the measuring electrode 110 and the referenceelectrode 120 are disposed, respectively. The via-conductor 122 iselectrically coupled to a base end of the reference electrode lead 121.

The reference electrode terminal 123 is printed on the major surface ofthe insulating ceramic base 200 which is flush with the measuringelectrode 110. The reference electrode terminal 123 is joinedelectrically to the via-conductor 122.

The surface of the insulating ceramic base 200 which is on the same sideas the surface of the solid electrolyte body 100 on which the measuringelectrode 110 is disposed will also be referred to as a measuringelectrode-side surface below. Similarly, the surface of the insulatingceramic base 200 which is on the same side as the surface of the solidelectrolyte body 100 on which the reference electrode 120 is disposedwill also be referred to as a reference electrode-side surface below. Onthe reference electrode-side surface, the heating element 140 is, asclearly illustrated in FIG. 5( c), printed. The heating element 140 ismade of a substantially C-shaped conductor which embraces at least threeof four sides of the reference electrode 120 at a constant interval daway from the solid electrolyte body 100 and/or the reference electrode120. The interval d is longer than or equal to 0.1 mm or shorter than orequal to 3 mm.

The heater leads 141 and 142 are printed on the reference electrode-sidesurface of the insulating ceramic base 200 in electric connection withthe ends of the heating element 140.

The via-conductors 143 and 144 are formed inside the holes 202 and 204of the insulating ceramic base 200 using the known vacuum printingtechniques. The holes 202 and 204 extend through the measuringelectrode-side surface and the reference electrode-side surface of theinsulating ceramic base 200. The via-conductors 143 and 144 areelectrically coupled to base ends of the heater leads 141 and 142,respectively.

The heater terminals 145 and 146 (which are also referred to as powersupply terminals) are printed on a base end portion of the measuringelectrode-side surface of the insulating ceramic base 200. The heaterterminals 145 and 146 are joined electrically to the via-conductors 143and 144, respectively.

The measurement gas chamber 170 is, as illustrated in FIGS. 4, 5(b), and5(d), of a substantially rectangular shape and covers the whole of thesurface of the measuring electrode 110. The measurement gas chamber 170is formed by applying a paste made of a mixture of binding agent andorganic solvent over the measuring electrode 110 and burning it out whenthe sensor/heater laminate sheet 20 is fired in the manner, as describedlater.

The diffusion resistance layer 180 is made from a diffusion layer-makingpaste which, after fired, becomes a porous layer. The diffusionlayer-making paste is made by blending alumina powder, resin powder, andbinder with each other and mixing organic solvent with it. The ratio ofsuch compositions is so selected that the porous layer has a givendegree of diffusion resistance. The diffusion layer-making paste isprinted over the whole of the surface of the measurement gas chamber 170so that it extends until an edge of the top end (i.e., a left end, asviewed in FIG. 4) of the insulating ceramic base 200.

The end of the diffusion resistance layer 180 is, as illustrated inFIGS. 4, 5(a), and 5(b), cut or grounded to form a tapered surface(i.e., the slant measurement gas inlet surface 181) which is not coveredwith the shield layer 190 and through which the measurement gas is to beintroduced into the diffusion resistance layer 180.

The slant measurement gas inlet surface 181 is inclined at a given angleto the length of the gas sensor element 10 to define an inlet openingwhich facilitates the ease with which a flow of the measurement gasmoving substantially perpendicular to the length of the gas sensorelement 10 is admitted into the diffusion resistance layer 180 and theninto the measurement gas chamber 170 which is lower in diffusionresistance than the diffusion resistance layer 180.

The slant measurement gas inlet surface 181 may be formed either beforeor after the sensor/heater laminate sheet 20 is wrapped about thecylindrical ceramic base 13 or after an assembly of the sensor/heaterlaminate sheet 20 and the cylindrical ceramic base 13 is fired.

The shield layer 190 is, as illustrated in FIGS. 4, 5(a), and 5(b),printed using an insulating paste such as alumina over the diffusionresistance layer 180 and the buffer layer 191. The shield layer 190 doesnot cover the terminals 145, 112, 123, and 146. The buffer layer 191 isprinted using an insulating paste such as alumina on the measurementelectrode-side surface of the insulating ceramic layer 200 in alignmentwith the diffusion resistance layer 180 in the lengthwise direction ofthe gas sensor element 10.

The shield layer 190 may be made using the alumina sheet SH₂₀₀.

In the manner, as described above, the planar sensor/heater laminatesheet 20 is produced which is a stack of the solid electrolyte body 100,the measuring electrode 110, the reference electrode 120, the heatingelement 140, the measurement gas chamber 170, the diffusion resistancelayer 180, the shield layer 190, the insulating ceramic layer 200, etc.

The cylindrical ceramic base 13 is formed by a hollow ceramic cylindermade of an insulating ceramic material such as alumina. The hollowceramic cylinder may be made using known extrusion-molding,injection-molding, CIP (Cold Isostatic Pressing), or HIP (Hot Isostaticpressing) techniques. The cylindrical ceramic base 13 may be shaped tobe 2.5 mm in outer diameter, 2.1 mm in inner diameter, and 50 mm inlength. The cylindrical ceramic base 13 has the closed end 133 that isthe top end of the gas sensor element 10 and the open end 134 that is toface the base end of the gas sensor 1. The cylindrical ceramic base 13has defined therein the cylindrical reference gas chamber 130 to whichthe air is admitted as the reference gas. The cylindrical ceramic base13 also has a rectangular hole or window 132 which passes through theside wall 131. The window 132 is located closer to the closed end thanto the open end 134.

The production of the cylindrical ceramic base 13 using the extrusionprocess is achieved by blending alumina power with binder, parting agent(also called mold release agent), and deionized water to produce a greenbody, extruding the green body into a hollow ceramic cylinder, cuttingthe hollow ceramic cylinder into a given length, closing one of openends of the hollow ceramic cylinder using a similar green body, dryingthe hollow ceramic cylinder to have an increased mechanical strength,and drilling the side window 132.

The production of the cylindrical ceramic base 13 using the injection,the CIP, or the HIP process is achieved by using a set of dies and acore. The use of the dies and the core enables the reference gas chamber130, the side wall 131, the side window 132, and the closed end 133 tobe formed simultaneously.

After the sensor/heater laminate sheet 20 and the cylindrical ceramicbase 13 are produced in the manner, as described above, thesensor/heater laminate sheet 20 is, as illustrated in FIG. 5( d),twisted or wrapped around the peripheral wall of the cylindrical ceramicbase 13 and then fired at a given temperature. For example, such anassembly is heated at 400 degree C. (Celsius, centigrade) for four hoursin the atmosphere to be degreased and then fired at approximately 1500degree C. for two hours to complete the gas sensor element 10.

When the sensor/heater laminate sheet 20 is wrapped about thecylindrical ceramic base 13, the reference electrode 110 is, as can beseen from FIG. 1( c), positioned in coincidence with the side window 132of the cylindrical ceramic base 13 in the radius direction thereof.

In the ceramic base wrapping process, it is advisable that a bondingpaste be applied to the reference electrode-side surface of theinsulating ceramic base 200 except the reference electrode 120 to gluethe sensor/heater laminate sheet 20 to the cylindrical ceramic base 200.The bonding paste may be made by dispersing alumina and binder inorganic solvent.

The wrapping of the sensor/heater laminate sheet 20 around thecylindrical ceramic base 13 may be achieved after the cylindricalceramic base 13 is dried, after the binder is removed from thecylindrical ceramic base 13, or after the cylindrical ceramic base 13 isfired temporarily or completely. The drying of the cylindrical ceramicbase 13 results in an increase in mechanical strength, thus facilitatingthe ease with which the sensor/heater laminate sheet 20 is wrapped aboutthe cylindrical ceramic base 13.

After the binder is removed from the cylindrical ceramic base 13, thecylindrical ceramic base 13 become porous and thus is impregnated withthe bonding paste, thereby resulting in a firm joint of the cylindricalceramic base 13 to the sensor/heater laminate sheet 20. This minimizesthe risk of delamination of the cylindrical ceramic base 13 and thesensor/heater laminate sheet 20.

When the cylindrical ceramic base 13 is fired temporarily at atemperature lower than that at which the assembly of the cylindricalceramic base 13 and the sensor/heater laminate sheet 20 is fired, itwill cause neck growth between alumina particles in the cylindricalceramic base 13, thereby resulting in an increase in mechanical strengthof the cylindrical ceramic base 13. This minimizes the probability ofbreakage of the cylindrical ceramic base 13 around which thesensor/heater laminate sheet 20 is wrapped.

The firing of the cylindrical ceramic base 13 will result in an increasein mechanical strength thereof, thus facilitating the ease with whichthe sensor/heater laminate sheet 20 is wrapped about the cylindricalceramic base 13.

The firing of the cylindrical ceramic base 13 will also result in adecrease in degree of shrinkage of the assembly of the cylindricalceramic base 13 and the sensor/heater laminate sheet 20 when fired tocomplete the gas sensor element 10. The thermal stress acting on thesensor/heater laminate sheet 20 will, therefore, be small, thus reducingthe probability of cracks in or delamination of the sensor/heaterlaminate sheet 20 from the cylindrical ceramic base 13. Care, however,should be taken not to over-fire the cylindrical ceramic base 13.

FIGS. 6( a) to 7(d) show modifications of the gas sensor element 10.

The heating element 140 is, as described above of a C-shape surroundingthe reference electrode 120, but may be printed, as illustrated in FIG.6( a), in the form of a bellows 140 a.

The slant measurement gas inlet surface 181 of the diffusion resistancelayer 180 serving as the gas inlet opening is defined by the tapered endsurface thereof, but however, side surfaces of the diffusion resistancelayer 180 may alternatively be, as illustrated in FIGS. 6( b), 6(c),6(d), and 6(e), tapered to form gas inlet openings through which themeasurement gas is admitted into the diffusion resistance layer 180. Thetapered side surfaces extend in the lengthwise direction of theinsulating ceramic base 200. The gas inlet openings are orientedsubstantially perpendicular to the length of the gas sensor element 10.

The measuring electrode terminal 112, the reference electrode terminal123, and the heater terminals 145 and 146 are, as illustrated in FIG. 2,collected on an area of the surface of the insulating ceramic base 200which occupies a portion of the circumference of the insulating ceramicbase 200, is aligned with the sensing mechanism, in other words, thelength of the diffusion resistance layer 180, and located closer to thebase end of the cylindrical ceramic base 13 than to the top end thereof,but however, may be, as denoted by numerals 112 b, 123 b, 145 b, and 146b in FIGS. 7( a) to 7(c), arrayed at regular or equi-intervals away fromeach other in the circumferential direction of the insulating ceramicbase 200 (i.e., the cylindrical ceramic base 13) and located closer tothe base end of the insulating ceramic base 200 than to the top endthereof. In this case, the terminals 112 b, 123 b, 145 b, and 146 b maybe, as illustrated in FIGS. 7( c) and 7(d), connected to the powersupply control circuit and the detector circuit, as described above,through substantially U-shaped spring connectors, as illustrated in FIG.7( d). the spring connectors are equipped with sets of a contact 113 band a terminal 114 b, a contact 124 b and a terminal 125 b, a contact147 b, and a terminal 149 b, a contact 148 b and a terminal 150 b,respectively. The contacts 113 b, 124 b, 147 b and 148 b are elasticallyplaced in electric contact with the terminals 112 b, 123 b, 145 b, and146 b on the insulating ceramic base 200, respectively. This structurefacilitates the ease with which the gas sensor element 10 is installedin the gas sensor 1 and improves the stability in electric connection ofthe terminals 112 b, 123 b, 145 b, and 146 b to the contacts 113 b, 124b, 147 b and 148 b against external mechanical vibration.

The heater terminals 145 b and 146 b are, as can be seen in FIG. 7( c),connected electrically to the power supply control circuit through theterminals 149 b and 150 b and conductive lines, respectively. The powersupply control circuit works to control the supply of electric power tothe heating element 140 and is equipped with a semiconductor switch SWsuch as a MOSFET, an SCR, or an IGBT and a driver DRV. The semiconductorswitch SW works to selectively establish or block the supply of powerfrom a storage battery BATT to the heater terminals 145 b and 146 b. Thedriver DRV works to control the operation of the semiconductor switch SWin a PWM control mode or a switching on/off control mode.

The measuring electrode 110 and the reference electrode 120 areelectrically connected to the detector circuit DTC through the terminals114 b and 125 b and conductive lines, respectively. The detector circuitDTC works as a gas concentration determining circuit to monitor adifference in electromotive force between the measuring electrode 110and the reference electrode 120 or an electric current flowing betweenthe measuring electrode 110 and the reference electrode 120 anddetermine the concentration of a specified component (e.g., O₂)contained in the measurement gas as a function of the monitoreddifference or current.

The disadvantages that gas sensor elements 10 z, 10 g, and 10 f whichare equipped with conventional structures and gas sensor elements 10 c,10 d, and 10 e which have technical limitations to beneficial effects asoffered by the structure of the gas sensor element 10 will be describedbelow with reference to FIGS. 8( a) to 13(b).

FIGS. 8( a), 8(b), and 8(c) illustrate, as a comparative example No 1,the gas sensor element 10 z having a typical planar structure as taughtin Japanese Patent First Publication No. H01-253649 discussed in theintroductory part of this application.

In FIGS. 8( a) to 8(c), the same reference numbers with a suffix “z”, asthose employed above, refer to similar or same parts, and explanationthereof in detail will be omitted here.

The planar solid electrolyte body 100 z has two opposed major surfaces.On one of the major surfaces, the measuring electrode 110 z, themeasuring electrode lead 111 z, the measurement gas chamber 170 z, thediffusion resistance layer 180 z, the shield layer 190 z, the bufferlayer 191 z, and the reference electrode terminal 123 z are formed. Onthe other major surface, the reference electrode 120 z and the referenceelectrode lead 121 z are formed. The solid electrolyte body 100 z hasformed therein the hole 101 z in which the via-conductor 122 z is formedto connect between the end of the reference electrode lead 121 z and thereference electrode terminal 123 z.

The reference gas chamber layer 131 z is stacked on the solidelectrolyte body 100 z. The planar insulating layer 150 z is alsostacked on the reference gas chamber layer 131 z to define the referencegas chamber 130 z along with the reference gas chamber layer 131 z. Theheater carrier layer 160 z on which the heating element 140 z, and theheater leads 141 z 142 z are formed is affixed to the insulating layer150 z. The heater carrier layer 160 z has formed therein holes 161 z and162 z in which via-conductors 144 z and 143 z are formed to connect theheater leads 141 z and 142 z and the heater terminals 145 z and 146 z,respectively.

The shield layer 181 z has the slant measurement gas inlet surfaces 181z formed on the sides thereof.

When the heating element 140 z is activated, thermal energy heatproduced by the heating element 140 z, as illustrated in FIG. 8( c),heats the air admitted into the reference gas chamber 130 z through theinsulating layer 150 z and is also transmitted to the solid electrolytebody 100 z through the air and the reference gas chamber layer 131 z.

The air exists in the reference gas chamber 130 z in the form of alayer. Such an air layer is high in thermal insulation and low inthermal conductivity (0.15 to 0.25 W/m·k).

The thermal energy produced by the heating element 140 z is partiallytransmitted to the solid electrolyte body 100 z through the insulatinglayer 150 z made of alumina whose thermal conductivity is high (20 to 30W/m·k) and the reference gas chamber layer 131 z. The sensing mechanismmade up of the measuring electrode 100 z, the reference electrode 120 z,etc. is, thus, heated by the thermal energy transmitted through thesolid electrolyte body 100 z which is made of zirconia whose thermalconductivity is low (2 to 3 W/m·k).

Most of the thermal energy produced by the heating element 140 z isfirst consumed in heating the air in the reference gas chamber 130 z.The air, as elevated in temperature thereof, is then transmitted to heatthe solid electrolyte body 100 z.

The radiant heat from the heating element 140 z is also transmitteddirectly to the solid electrolyte body 100 z. Yttria-stabilized zirconiawhich is typically used as material of the solid electrolyte body 100 zis white in color and thus reflects most of the radiant heat. Thethermal energy produced by the heating element 140 is, therefore, notused in activating the solid electrolyte body 100 z immediately afterthe heating element 140 is energized, thus resulting in a delay inbringing the gas sensor element 10 z into a condition to measure theconcentration of gas correctly.

The heating element 140 z is, as described above, interposed between theinsulating layer 150 z and the heater carrier layer 160 z to make aheating mechanism. The sensing mechanism (i.e., the solid electrolytebody 100 z, the measuring electrode 110 z, the reference electrode 120z, and the reference gas chamber 130 z) and the heating mechanism (i.e.,the heating element 140 z, the insulating layer 150 z, and the heatercarrier layer 160 z) are on opposite sides of the reference gas chamber130 z. The temperature of the heating mechanism is, thus, elevated morethan that of the sensing mechanism. This will cause the heatingmechanism to be greater in degree of thermal expansion than the sensingmechanism, thus resulting in tensile stress acting on the outer surfaceof the heating mechanism, which may lead to cracks in the insulatinglayer 150 z and the heater carrier layer 160 z.

In order to minimize the risk of such cracks, the insulating layer 150 zand the heater carrier layer 160 z may be thickened to absorb thethermal stress, but it results in an increase in overall size of the gassensor element 10 z, in other words, an increase in volume of the gassensor element 10 z to be heated. This also results in a delay inactivating the gas sensor element 10 z.

The solid electrolyte body 100 z is, as illustrated in FIG. 8( a),entirely planar and has the electric conductivity. The measuringelectrode lead 111 z, the reference electrode lead 121 z, the measuringelectrode terminal 112 z, the via-conductor 122 z, and the referenceelectrode terminal 123 z are formed on the solid electrolyte body 100 z.Application of voltage between the measuring electrode terminal 112 zand the reference electrode terminal 123 z, therefore, causes electriccurrent to flow through the solid electrolyte body 100 z, which resultsin a decrease in accuracy in measuring the concentration of gas.

FIGS. 9( a), 9(b), and 9(c) illustrate, as a comparative example No 2,the gas sensor element 10 g which is similar in structure to the one, astaught in Japanese Patent First Publication No. H01-253649 discussed inthe introductory part of this application.

In FIGS. 9( a) to 9(c), the same reference numbers with or without asuffix “g”, as those employed above, refer to similar or same parts, andexplanation thereof in detail will be omitted here.

The solid electrolyte body 100 of the gas sensor element 10 of theembodiment is, as illustrated in FIG. 4, embedded in the insulatingceramic base 200, while the solid electrolyte body 100 g of the gassensor element 10 g is planer and wrapped directly about the cylindricalinsulating base 13. Specifically, the solid electrolyte body 100 g hasaffixed on opposed major surfaces thereof the measuring electrode 110 g,the measuring electrode lead 111 g, the measuring electrode terminal 112g, the reference electrode 120 g, the reference electrode lead 121 g,the via-conductor 122 g, and the reference electrode terminal 123 g. Thesolid electrolyte body 100 g is wrapped around the cylindricalinsulating base 13 b with the reference electrode 120 g facing the hole132 g. The heating element 140 g, the heater leads 141 g and 142 g, andthe heater terminals 145 g and 146 g are disposed through the insulatinglayer 150 g on an area of the solid electrolyte body 100 around themeasuring electrode 110 g. The opening 151 g is formed in a portion ofthe insulating layer 150 which coincides with the measuring electrode110 g. The opening 151 g defines, as illustrated in FIG. 9( b), themeasurement gas chamber 170 g. The diffusion resistance layer 180 g isdisposed on the insulating layer 150 g to cover the measuring electrode110 g exposed to the opening 151 g. The shield layer 190 g is laid overthe diffusion resistance layer 180 g. These arrangements enables theinterval (i.e., the insulating interval d) between the measuringelectrode 100 g and the inner edge of the heating element 140 to beshortened to accelerate the activation of the solid electrolyte body 100g, but ensuring a desired degree of electric insulation between theheating element 140 g and the solid electrolyte body 100 g requires anincrease in thickness t₁₅₀ of the insulating layer 150 g to set theinsulating interval d to be greater than or equal to, for example, 100μm.

The solid electrolyte body 100 g is wrapped around the wholecircumference of the cylindrical ceramic base 13. In other words, theinsulating layer 150 g is wrapped around the whole of the circumferenceof the cylindrical ceramic base 13. The shield layer 190 g covers thewhole of the circumference of the insulating layer 150 g. The slantmeasurement gas inlet surface 181 g is, thus, formed inevitably on thetop end of the diffusion resistance layer 180 g.

The outer diameter of the gas sensor element 10 g is, therefore,increased, thereby resulting in an increase in entire thermal capacityof the gas sensor element 10 g.

The heating element 140 g, as can be seen from FIG. 9( c), heats thesolid electrolyte body 100 g from the outside in the circumferentialdirection of the solid electrolyte body 100 g. The thermal energyproduced by the heating element 140 g, therefore, like in thecomparative example No. 1, passes through the solid electrolyte body 100g which is low in thermal conductivity and then reaches two sides of thesensing mechanism.

The heating element 140 g is located close to the outer periphery of thegas sensor element 10 g, so that lots of thermal energy diffuses to themeasurement gas around the gas sensor element 10 g, thus resulting inlack of the thermal energy to heat the air in the reference gas chamber130.

The structure of the comparative example No. 2 is, therefore, low inenergy efficiency and takes time to activate the sensing mechanismcompletely.

FIGS. 10( a), 10(b), and 10(c) illustrate, as a comparative example No3, the gas sensor element 10 f. In FIGS. 10( a) to 10(c), the samereference numbers with or without a suffix “f”, as those employed above,refer to similar or same parts, and explanation thereof in detail willbe omitted here.

The gas sensor element 10 f is similar to the gas sensor element 10 inthat the solid electrolyte body 100 is embedded in the insulatingceramic base 200 f, and the heating element 140 f is located on thesurface of the insulating ceramic base 200 f on which the referenceelectrode 120 is disposed, but however, the reference gas chamber 130 fis defined by a stack of the C-shaped reference gas chamber layer 131 fand the planar insulating layer 150 f without use of the cylindricalceramic base 13.

Accordingly, the gas sensor element 10 f is, as can be seen in FIG. 10(b), identical in longitudinal section with the gas sensor element 10,but different, as can be seen in FIG. 10( c), in transverse section fromthe gas sensor element 10. Quick thermal activation of the solidelectrolyte body 100 is, therefore, thought of as being achieved, likethe gas sensor element 10, through the insulating ceramic base 200 fwhich is high in thermal conductivity by mounting the heating element140 on the same side of the insulating ceramic base 200 f as thereference electrode 120.

However, because the heating element 140 extends outside the sides ofthe reference electrode 120 f on the insulating ceramic base 200 f whichis planar, the gas sensor element 10 f has an increased width. Thisresults in increased sensitivity of the gas sensor element 10 f tothermal stress arising from, for example, splashing with water, whichdecreases the durability of the gas sensor element 10 f.

FIGS. 11( a), 11(b), and 11(c) illustrate, as a comparative example No4, the gas sensor element 10 c. In FIGS. 11( a) to 11(c), the samereference numbers with or without an suffix “c”, as those employedabove, refer to similar or same parts, and explanation thereof in detailwill be omitted here.

The gas sensor element 10 f is different from the gas sensor element 10of FIG. 1 only in that the insulating interval d between the edge of thesolid electrolyte body 100 and the heating element 140 c is shorter than0.1 mm that is a lower limit of a set range of the insulating interval din the gas sensor element 10.

The structure of the gas sensor element 10 c is identical in effect toactivate the sensing mechanism quickly with that of the gas sensorelement 10, but the decreased insulating interval d results in leakageof current from the heating element 140 c when energized, which leads toinstability in operation of the gas sensor element 10 c.

FIGS. 12( a), 12(b), and 12(c) illustrate, as a comparative example No5, the gas sensor element 10 d. In FIGS. 12( a) to 12(c), the samereference numbers with or without an suffix “d”, as those employedabove, refer to similar or same parts, and explanation thereof in detailwill be omitted here.

The structure of the gas sensor element 10 d is similar to that of thegas sensor element 10 of FIG. 1 in that the solid electrolyte body 100is embedded in the insulating ceramic base 200 with the measuringelectrode 110 and the reference electrode 120 affixed to opposed majorsurfaces thereof and wrapped around the cylindrical ceramic base 13, butdifferent in that the heating element 140 d is disposed on the surfaceof the insulating ceramic base 200 which is on the same side as themeasuring electrode 110, and the insulating layers 192 d and 193 d areprinted over the heating element 140 d as protective layers whichisolate the heating element 140 d from the measurement gas. Theinsulating layers 192 d and 193 d are 20 μm in thickness.

The quick thermal activation of the solid electrolyte body 100 is,therefore, thought of as being achieved by shortening the insulatinginterval between the measuring electrode 110 and the heating element 140d, but the insulating layers 192 d and 193 d printed to cover theheating element 140 d are thin, thus resulting in greater concern aboutthe deterioration of operation of the heating element 140 d due toexposure to poisons in the measurement gas, as having penetratedpinholes in the insulating layers 192 d and 193 d.

The heating element 140 d is, like in the comparative example No. 2,located close to the outer periphery of the gas sensor element 10 d, sothat lots of thermal energy produced by the heating element 140 ddiffuses to the measurement gas around the gas sensor element 10 d, thusresulting in lack of the thermal energy to heat the air in the referencegas chamber 130.

FIGS. 13( a), 13(b), and 13(c) illustrate, as a comparative example No6, the gas sensor element 10 e. In FIGS. 13( a) to 13(c), the samereference numbers with or without a suffix “e”, as those employed above,refer to similar or same parts, and explanation thereof in detail willbe omitted here.

The structure of the gas sensor element 10 e is similar to that of thegas sensor element 10 of FIG. 1 in that the solid electrolyte body 100is embedded in the insulating ceramic base 200 with the measuringelectrode 110 and the reference electrode 120 affixed to opposed majorsurfaces thereof and wrapped around the cylindrical ceramic base 13, butdifferent in that the heating element 140 e is disposed on the surfaceof the insulating ceramic base 200 which is on the same side as themeasuring electrode 110, and the insulating layers 192 e and 193 e areprinted over the heating element 140 d as protective layers whichisolate the heating element 140 d from the measurement gas. Theinsulating layers 192 e and 193 e are formed by the doctor bladetechniques to have a thickness of 220 μm. The quick thermal activationof the solid electrolyte body 100 is, therefore, thought of as beingachieved by shortening the insulating interval between the measuringelectrode 110 and the heating element 140 e. Additionally, theinsulating layers 192 e and 193 e are thicker than the insulating layers192 d and 193 in the comparative example No. 5, thus minimizing the riskof deterioration of operation of the heating element 140 d due toexposure to poisons in the measurement gas, as having penetrate pinholesin the insulating layers 192 e and 193 e. However, the heating element140 d is, like in the comparative example Nos. 2 and 5, located close tothe outer periphery of the gas sensor element 10 e, so that lots ofthermal energy produced by the heating element 140 e diffuses to themeasurement gas around the gas sensor element 10 e, thus resulting inlack of the thermal energy to heat the air in the reference gas chamber130.

The increased thickness of the insulating layers 192 e and 193 e,however, undesirably absorbs the thermal energy produced by the heatingelement 140 e, thus resulting in a delay in activating the gas sensorelement 10 e.

FIGS. 14( a), 14(b), and 14(c) illustrate, as a comparative example No7, the gas sensor element 10 h. In FIGS. 14( a) to 14(c), the samereference numbers with or without a suffix “h”, as those employed above,refer to similar or same parts, and explanation thereof in detail willbe omitted here.

The gas sensor element 10 h is different from the gas sensor element 10of FIGS. 1( a) to 1(c) only in that the insulating interval d betweenthe edge of the solid electrolyte body 100 and the heating element 140 his 3.5 mm which is greater than an upper limit of the set range of theinsulating interval d in the gas sensor element 10 by 0.5 mm.

The increased insulating interval d results in an increase in timerequired to activate the gas sensor element 10 h completely.

We performed tests, as discussed below, to evaluate the beneficialeffects, as offered by the structure of the gas sensor element 10.

We prepared test samples identical in structure with the gas sensorelement 10 and the comparative example Nos. 1 to 6 (i.e., the gas sensorelements 10 z, 10 g, 10 f, 10 c, 10 d, and 10 e and analyzed three testitems: 1) activation time that is the time required to activate the testsamples, 2) heater durability, and 3) thermal stress breakage.

Activation Time

We applied 6.5V to the heating element of each test sample, also applied0.4V between the measuring electrode and the reference electrode, andmeasured the time that elapsed before a value of resulting currentflowing through the measuring and reference electrode falls within ±2%of a constant or steady current. We found that less than 6 seconds isacceptable.

Heater Durability

We applied 6.5V to the heating element for 1,000 hours and then measureda change in resistance of the heating element. We determined that theheating element whose resistance didn't change is acceptable.

Thermal Stress Breakage

We energized the heating element and applied drops of water to each testsample. We measured a total volume of water applied until the testsample was broken. We defined such a total volume of water applied tothe test sample of the comparative example No. 1 (i.e., the gas sensorelement 10 z) as a reference volume unit 10 and calculated the totalvolume of water applied to each test sample relative to the referencevolume unit 10. Note that the higher the mechanical strength of eachtest sample, the more the total volume of water will be.

We shows results of the above tests in TABLE 1, as appears on thefollowing page.

We found that, for the test sample of the gas sensor element 10 of theembodiment, the activation time was four seconds, the value of theresistance of the heating element was 1.9Ω either before or after the1000 hour-durability test, and the applied total volume of water is muchsmaller than the reference unit 10. It is, thus, found that the gassensor element 10 is substantially identical in degree of the heaterdurability with the comparative example No. 1, but excellent in theactivation time and resistance to the thermal stress breakage. We gave agood overall rating “o” for the gas sensor element 10 in TABLE 1.

Comparative Example No. 1

The activation time of the comparative example No. 1 (i.e., the gassensor element 10 z) was ten seconds. However, we evaluated that anactivation time of less than 10 seconds was required for beingacceptable. The value of the resistance of the heating element was 2.0Ωeither before or after the 1000 hour-durability test,

Comparative Example No. 2

The activation time of the comparative example No. 2 (i.e., the gassensor element 10 g) was six seconds which is less than that of thecomparative example No. 1, but longer than that of the gas sensorelement 10. The value of the resistance of the heating element was 1.9Ωand 2.0Ω before and after the 1000 hour-durability test, respectively,which are almost identical with those of the gas sensor element 10. Theapplied total volume of water is smaller than that of the gas sensorelement 10 even though the outer diameter is greater than that of thegas sensor element 10. This is because the heating element is locatedcloser to the outer surface of the gas sensor element 10 g, so that thetemperature of the outer surface is elevated, thus resulting in anincrease in thermal stress acting the surface of the gas sensor element10 g when splashed with water. We found that the activation time of thecomparative example No. 2 is improved compared to the previouscomparative example, but the resistance to the thermal stress breakageis lower than that of the comparative example No. 1 and thus gave a badoverall rating “x” for it in TABLE 1.

Comparative Example No. 3

The activation time of the comparative example No. 3 (i.e., the gassensor element 10 f) was four seconds which is approximately the same asthe gas sensor element 10. The value of the resistance of the heatingelement was 1.9Ω and 2.0Ω before and after the 1000 hour-durabilitytest, respectively, which are almost identical with those of the gassensor element 10. The applied total volume of water is approximatelyhalf that of the comparative example No. 1. This is because the gassensor element 10 f is planar and wide and thus low in resistance to thethermal stress.

We found that the activation time of the comparative example No. 3 isgreatly improved compared to comparative example No. 1, but theresistance to the thermal stress breakage is much lower than that of thecomparative example No. 1 and thus gave a bad overall rating “x” for itin TABLE 1.

Comparative Example No. 4

The activation time of the comparative example No. 4 (i.e., the gassensor element 10 c) was four seconds which is approximately the same asthe gas sensor element 10, but we observed the leakage of current fromthe heating element in the form of a significant electrical noise. Thecurrent output was not kept in ±2% of the steady current. The value ofthe resistance of the heating element was 1.9Ω either before or afterthe 1000 hour-durability test. The applied total volume of water isapproximately the same as that of the gas sensor element 10.

We found that the activation time and the resistance to the thermalstress breakage are greatly improved, but the noise is great, and thereliability in operation of the gas sensor element 10 c is notacceptable. We therefore gave a bad overall rating “x” for it in TABLE1.

Comparative Example No. 5

The activation time of the comparative example No. 5 (i.e., the gassensor element 10 d) was four seconds which is approximately the same asthat of the gas sensor element 10, but the value of the resistance ofthe heating element changed greatly from 2.0Ω to 28Ω after the 1000hour-durability test. This is because the protective layers 192 d and193 d are thin, which accelerates the deterioration of the heatingelement 140 d. The resistance to the thermal stress breakage wasapproximately the same as that of the gas sensor element 10.

We found that the activation time and the resistance to the thermalstress breakage of the comparative example No. 5 are greatly improved,but the heater durability is low, and thus gave a bad overall rating “x”for it in TABLE 1.

Comparative Example No. 6

The activation time of the comparative example No. 6 (i.e., the gassensor element 10 e) was six seconds which is longer than that of thegas sensor element 10. This is because the heating element 140 e islocated closer to the outer surface of the gas sensor element 10 e, sothat the thermal energy produced by the heating element 140 e isabsorbed by the protective layers 192 e 193 d. The value of theresistance of the heating element was 1.9Ω and 2.0Ω before and after the1000 hour-durability test, respectively, which are almost identical withthose of the gas sensor element 10. The resistance to the thermal stressbreakage was approximately the same as that of the gas sensor element10.

We found that, for the comparative example No. 6, the resistance to thethermal stress breakage is greatly increased, the activation time isslightly improved, and the heater durability is acceptable and thus gavean average overall rating “A” for it in TABLE 1.

Comparative Example No. 7

The activation time of the comparative example No. 7 (i.e., the gassensor element 10 h) was six seconds which is longer than that of thegas sensor element 10. This is because the heating element 140 h islocated far away from the solid electrolyte body.

The gas sensor element 10 h is cylindrical in shape. A slight increasein insulating interval d, thus, results in an increase in volume of aportion of the gas sensor element 10 h through which the thermal energyproduced by the heating element 140 h is transmitted to the solidelectrolyte body 100. Such a volume increase is thought of ascontributing to the increase in the activation time.

The value of the resistance of the heating element was 1.9Ω and 2.0Ωbefore and after the 1000 hour-durability test, respectively, which arealmost identical with those of the gas sensor element 10. The resistanceto the thermal stress breakage was approximately the same as that of thegas sensor element 10.

We found that, for the comparative example No. 7, the resistance to thethermal stress breakage is greatly increased compared with comparativeexample No. 1, the activation time is slightly improved, and the heaterdurability is acceptable and thus gave an average overall rating “Δ” forit in TABLE 1.

As apparent from the above discussion, the gas sensor element 10 of theembodiment is made up of the cylindrical ceramic base 13 and thesensor/heater laminate sheet 20 stacked on the cylindrical ceramic base13. The sensor/heater laminate sheet 20 has the solid electrolyte body100 disposed in the insulating ceramic base 200. The solid electrolytebody 100 works to conduct at least a given ion (e.g., an oxygen ion) ofgas. The cylindrical ceramic base 13 has the open end 134 (i.e., theinlet for the air) and the closed end 133. The cylindrical ceramic base13 defines therein the reference gas chamber 130 and has formed thereinthe window 132 to which the reference electrode 120 affixed to one ofthe major surfaces of the solid electrolyte body 100 is exposed. Themeasuring electrode 110 is affixed to the other major surface of thesolid electrolyte body 100 and exposed to the measuring gas. The heatingelement 140 is disposed on the surface of the insulating ceramic base200 on the same side as that of the solid electrolyte body 100 on whichthe reference electrode 120 is mounted. The heating element 140 works toactivate the solid electrolyte body 100 to produce a signal as afunction of the concentration of a given component of the measurementgas. The heating element 140 is located near the longitudinal center ofthe gas sensor element 10. This achieves quick activation of the gassensor element 10 and ensures a required degree of durability thereof.The insulating interval d is, as described above, the shorter of aminimum distance between the inner edge of the heating element 140 andthe outer edge of the reference electrode 120 and a minimum distancebetween the inner edge of the heating element 140 and the outer edge ofthe solid electrolyte body 100 and selected to be longer than or equalto 0.1 mm and shorter than or equal to 3 mm.

TABLE 1 RESIS- TANCE APPLIED ACTIVATION TIME CHANGE WATER RATINGEmbodiment 4 0.0 Ω 41 ◯ Comparative Example No. 1 10 0.0 Ω 10 XComparative Example No. 2 6 0.1 Ω 17 X Comparative Example No. 3 4 −0.1Ω  6 X Comparative Example No. 4 4 0.0 Ω 40 X Comparative Example No. 54 26.0 Ω  39 X Comparative Example No. 6 6 −0.1 Ω  37 Δ ComparativeExample No. 7 6 0.1 Ω 37 Δ

While the present invention has been disclosed in terms of the preferredembodiments in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiments witch can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. A gas sensor element production methodcomprising: forming an insulating ceramic member which is planar andmade of an electrically insulating ceramic material, the insulatingceramic member having opposed surfaces and a through hole; forming asolid electrolyte body in the through hole of the insulating ceramicmember, the solid electrolyte body being made of a ceramic materialwhich conducts at least a given ion and having a first major surface anda second major surface; forming a measuring electrode disposed on thefirst major surface of the solid electrolyte body to be exposed to thegas; forming a reference electrode disposed on the second major surfaceof the solid electrolyte body to be exposed to a reference gas;arranging a heating element disposed on one of the opposed surfaces ofthe insulating ceramic member on the same side as the second majorsurface of the solid electrolyte body, the heating element working toactivate the solid electrolyte body; preparing a hollow cylindricalceramic member which is made of an electrically insulating ceramicmaterial and has a closed end and an open end, the hollow cylindricalceramic member also having a window formed in a peripheral surfacethereof; wrapping the insulating ceramic member around the hollowcylindrical ceramic member with the reference electrode facing thewindow of the hollow cylindrical ceramic member; and firing the hollowcylindrical ceramic member around which the insulating ceramic member iswrapped.
 2. A gas sensor element production method as set forth in claim1, wherein said forming steps of the insulating ceramic member and thesolid electrolyte body prepare a stack of a planar solid electrolytematerial and a planer insulating ceramic material and punch the stack tomake the through hole in the insulating ceramic member and the solidelectrolyte body cut from the planar solid electrolyte material to havea size substantially identical with that of the through hole andsimultaneously to place the solid electrolyte body in the through hole.